1. Field of the Invention
The present invention relates to a toner, which is suitably used in electrophotography, electrostatic recording, and electrostatic printing, and also relates to an image forming apparatus using such toner.
2. Description of the Related Art
In recent years, formation of images having long shelf life is possible at high speed and low energy in the field of electrophotographic image formation technology. In addition, competitions in developing a color image forming apparatus that gives high image quality have been severe.
In order to meet the need for high image quality, especially full-color image quality, it has been considered that a size of a toner is reduced to accurately reproduce a latent image. Moreover, there has been an attempt to achieve high image quality by controlling a shape of a toner. As a result of this, reproducibility of dots or fine lines is improved, and a pile height (a thickness of an image layer) can be kept low, and therefore an achievement of higher image quality can be expected. Further, for high speed and low energy consuming image formation, required is a toner having the reduced fixing temperature itself, the reduced fixing temperature of the toner during usage, and high lubricity.
To solve the aforementioned problems, it has been widely studied in a structure and a molecular weight of a binder resin, and characteristics of a releasing agent, but heat resistance of a toner and stress resistance of a toner have a relationship of trading-off. In the case where a toner having small particle diameters is used, non-electrostatic adhesion force between a toner and an electrophotographic photoconductor, or between the toner with the electrophotographic photoconductor and an intermediate transfer member increases, and therefore a problem associated with stress resistance of a toner tends to occur. Regarding cleaning property of a toner, it has been known in the art that adhesion force of the toner increases as the particle size of the toner reduces, which lowers the cleaning property.
As for a method for removing an untransferred toner, there are various cleaning systems, such as cleaning blade, fur brush cleaning, and magnetic brush cleaning, but a method for using a cleaning blade is mainly used. This cleaning system is mainly to bring an elastic blade in contact with a photoconductor with an appropriate pressure, but use of a toner meting the current need of high image quality (small particle size, controlled shape, low temperature fixing) in this system causes that the toner passes through the blade, which results in cleaning failures. Studies have been conducted to increase the contact press of the blade and design an appropriate shape of the contact part to prevent the cleaning failures. By increasing the contact press, friction heat is generating in the cleaning unit. Especially a releasing agent having a low melting point or the like is bleed out from the toner with the influence of the local heat, and therefore the residual toner from transferring causes fusion by the friction heat, which becomes a factor of so-called filming.
As a method for solving the aforementioned problems, disclosed is a toner to which a lubricant component, such as a fatty acid metal salt, is added as an external additive of the toner for improving cleaning property (see Japanese Patent Application Laid-Open (JP-A) Nos. 60-198556, 61-231562, and 61-231563).
Moreover, JP-A No. 2010-79242 discloses a toner in which particle diameters and a libration ratio of fatty acid metal salt particles are defined, and discloses that cleaning property, occurrence of fogging, and reduction of image density can be improved by setting the libration ratio of the fatty acid metal salt particles to the range of 1.0% to 25.0%. In accordance with this method, however, it is difficult to maintain cleaning property of the toner in view of the cleaning property, under the circumstance that there is a strong need for a toner of a long shelf life and for use in high speed printing, and the toner satisfying the low temperature fixing ability has a problem that a problem, such as filming, may occur during usage for a long period.
Moreover, JP-A No. 2006-154387 discloses an image forming apparatus in which a libration rate of an external additive liberated from a surface of a toner for use is 5% to 50%, and a fatty acid metal salt is applied on a photoconductor. In accordance with this method, it is disclosed that uneven abrasion of a photoconductive layer of the photoconductor is prevented and a long service life of the photoconductor is maintained. However, in this method, the libration rate of the external additive is high, and therefore there is a possibility that the librated external additive may damage the photoconductor to thereby cause filming.
Accordingly, there are currently needs for a toner having all of high speed printing, long shelf-life, and low temperature fixing ability that are required for electrophotographic image formation, and an image forming apparatus, which uses such toner, can maintain excellent cleaning ability over a long period, and can stably provide high quality images.